1. Technical Field of the Invention
This invention relates generally to audio amplifiers, and more specifically to an improvement in power efficiency in apparatus for driving a common load terminal bias in a stereo transducer system (a configuration common in headphone amplifiers where standard jacks tie both speaker commons together).
2. Background Art
An audio loudspeaker includes a pair of electrical input terminals, conventionally identified as the + terminal and the − terminal (also referred to as the common terminal or the ground terminal). The loudspeaker is driven with a zero average alternating current voice signal from an amplifier which is coupled to the terminals. In order to avoid having a DC offset over time, the + terminal is alternatingly provided with a voltage which is positive and then negative with respect to the − terminal. In some systems, the − terminal is held at a 0v ground potential, and the + terminal is fed a signal that, for example, swings between a maximum of +5v and a minimum of −5v. In other systems, the + signal swings between e.g. a maximum of +10v and a minimum of 0v, and the − terminal is held at 5v. Or, more precisely, the − terminal is held at Vcc/2, where Vcc is the maximum voltage that could be applied at the + terminal. In either case, the voltage at which the − terminal is held may be referred to as a “bias voltage”.
In most portable music systems with headphones, the − terminals of the left and right loudspeakers (e.g. headphones or ear buds) are coupled together at a common node, and the + and − terminals have very large ac coupling capacitors in series with them to drop VCC/2 of voltage. The − terminal is then tied to 0V. These coupling capacitors are very large and expensive, both of which are undesirable traits. An alternating solution that has been used is to tie the common − terminals together and to apply the VCC/2 or other bias voltage at this common node. In either configuration, the left and right loudspeakers' + inputs are then driven with the left and right voice signals, respectively.
A variety of circuit schemes have been used to generate the common bias voltage.
FIG. 1 illustrates an audio system 10 using a very simple common bias voltage scheme. The audio system includes a left loudspeaker SL and a right loudspeaker SR. The audio system includes a left amplifier AL which drives the left speaker and a right amplifier AR which drives the right amplifier. The − terminals of the loudspeakers are coupled together at a common node, and a suitably sized capacitor C1 is coupled between the common node and ground.
This system is power efficient but, unfortunately, requires a large, expensive capacitor. The capacitor is a high-pass filter with regard to the residual signal left across the speaker terminals; if the capacitor is too small, it will cut off audible low frequencies, preventing them from reaching the loudspeakers.
FIG. 2 illustrates an audio system 20 using a third amplifier A3 to provide the bias voltage at the common node. The + input of the third amplifier is tied to a voltage divider which includes matched resistors R1 and R2 coupled in series between Vcc and ground, with a capacitor C2 coupled from the voltage divider to ground to remove noise from the voltage divider. The third amplifier is coupled in negative feedback fashion, as shown.
This system requires only a small (and optional) capacitor, rather than the large capacitor of FIG. 1. Unfortunately, the voltage divider and the third amplifier consume non-trivial amounts of power, significantly reducing the overall power efficiency of the amplifier section of the system.
FIG. 3 illustrates a similar audio system 30 using Class G amplifiers. Unlike Class A, B, and AB amplifiers, which are single supply meaning that only a single pair of power rails (Vcc and ground) are provided to them, Class G amplifiers have two or more pairs of power rails, between which they switch, depending on the instantaneous voltage level of the input signal. The designer may elect to use Class G amplifiers, to minimize power wasted in the amplifier section of the system.
As shown, a Class G amplifier AG can be used to provide the bias voltage at the common node. This is independent of whether the left and right power amplifiers are also Class G. With the output of the third amplifier being very stable at Vcc/2, the third amplifier will typically be operating with its closest-to-center voltage rail pair selected, minimizing its power consumption.
Unfortunately, its power consumption will still be non-trivial, even with a stable Vcc/2 output.
What is needed is an improved amplifier section which uses an even lower-power circuit for providing the bias voltage at the loudspeakers' common node.
FIG. 4 illustrates a high efficiency charge pump circuit 40 which is known in analog and power semiconductor design. The charge pump is built around a flying capacitor CF and a switching device. The switching device has a first switch S1 coupled to Vcc and controlled by a first switch input signal PHI1, a second switch S2 controlled by the first switch input signal, a third switch S3 controlled by a second switch input signal PHI2, and a fourth switch S4 coupled to ground and controlled by the second switch input signal. The inputs of the second and third switches are coupled to an output node 50P. A second capacitor C50P is coupled between the output node and ground. When the first and second switch control signals are operated correctly (typically as binary clock signals 180° out of phase, with non-overlapping clocks), Vcc/2 is generated at the output node.
The charge pump is one example of a high efficiency converter. Another example is an inductive switching supply such as a buck converter, which is very well known in the art of power semiconductor design.